Process and composition for inhibiting the polymerization of cyclopentadiene compounds

ABSTRACT

A process for inhibiting the polymerization of cyclopentadiene compounds (B) by contacting the cyclopentadiene compound with a quinone methide compound (A) of structure (I), 
     
       
         
         
             
             
         
       
     
     Compositions (AB) comprising (A) and (B) are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 14/217,588filed on Mar. 18, 2014, claiming priority to German Application No.102013204950.1, filed Mar. 20, 2013, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to a composition comprising quinone methidesuseful for stabilizing cyclopentadiene compounds such as, for example,cyclopentadiene and dicyclopentadiene. It relates to a process forinhibiting the polymerization of cyclopentadiene compounds. Applicationmay be possible in all process streams comprising cyclopentadienecompounds.

DESCRIPTION OF THE BACKGROUND

Cyclopentadiene (=CPD) is a very reactive molecule which dimerizes todicyclopentadiene (=DCPD) even at low temperatures, via a Diels-Alderreaction. For this reason the dimeric form is also the commerciallyavailable form of cyclopentadiene. The monomer can be restored by aretro-Diels-Alder reaction at high temperatures. The reversibledimerization of CPD to DCPD can thus be described as depictedhereinbelow in reaction equation <1> (the forward reaction from CPD toDCPD is the dimerization, which is preferential at low temperatures; thereverse reaction from DCPD to CPD is the cleavage, which is preferentialat high temperatures, i.e. T>155° C.):

However, CDP and DCDP have a tendency to polymerize. This can beexplained by various mechanisms. These include inter alia thepossibility of further Diels-Alder reactions; on the other hand,free-radical polymerizations also take place. It is also possible toimagine mixed polymerization mechanisms. The Diels-Alder polymerizationis shown in reaction equation <2>.

This high tendency for CPD and its compounds to polymerize can lead to avariety of problems in all (di)cyclopentadiene-containing processstreams as well as with the specific production of cyclopentadienemonomer.

Cyclopentadiene compounds such as cyclopentadiene and dicyclopentadieneare thus present in some process streams, for example in pyrolysisgasoline, as secondary components, and can react with themselves orother vinyl-containing monomers in polymerization reactions. Theseundesired polymerization reactions occur particularly at hightemperatures and can lead to deposits in the plants. The consequence ofthis is a reduced heat transfer and hence a reduced productivity. If thedeposits lead to blockages, unscheduled cleaning of the plant has to becarried out, which leads to interruptions in the manufacturingoperation. Every outage adds costs due to repair and cleaning, but inparticular also due to the manufacturing outage itself. Avoidance ofsuch outages is therefore a constant objective.

The described problems due to undesired polymerization occur not just inprocess streams comprising cyclopentadiene compounds as secondarycompounds, but also and particularly in the production ofcyclopentadiene itself. As previously noted, cyclopentadiene is obtainedindustrially by cleaving (“cracking”) dicyclopentadiene (in accordancewith the high-T reverse reaction in the above reaction equation <1>).The cracking of dicyclopentadiene can take place not only in the liquidphase but also in the gas phase as described by Z. Cai, B. Shen, W. Liu,Z. Xin, and H. Ling (Energy & Fuels 2009, 23, 4077-4081). Particularlythe liquid phase version of cracking is prone to the problem of oligomerdeposits. Again, reduced heat transfer, reduced productivity and in theextreme case even blockage of plant components can occur, necessitatingshutdowns and cleans. Temperatures of not less than 155° C. are neededto crack dicyclopentadiene to cyclopentadiene. These temperatures leadto the very rapid formation of cyclopentadiene oligomers and polymers,which raise the viscosity and thereby make stirring more difficult, andwhich form deposits and inhibit effective heat transfer, making thereaction more difficult to carry out and entailing yield losses.

Various strategies are described to control the formation of oligomersin the cracking of dicyclopentadiene. The most widely used option is toadd an inert solvent as a bottoms diluent. Long-chain hydrocarbons areused for this in particular. The solvent does not prevent thepolymerization, but slows it, by reducing the concentration of thereactive component. The advantage of such use of solvent is that theoligomers formed dissolve in the solvent and so do not form deposits, asa result of which the reaction mixture remains workable. Moreover, usinga solvent can be used to shorten the period of thermal exposure. Thisadvantage is described by Ammannati et al. (WO20101020549). Diphenylether is used therein as inert solvent. Robota (DE1951320; GB1261565)describes a cracking process wherein a paraffinic hydrocarbon oil isemployed as a solvent.

On the other hand, the employment of an inert solvent entails thedisadvantage that a portion of the reactor volume is already occupied bythe solvent, so the volume-time yield is reduced by the addition of asolvent.

Another conventionally known method to inhibit undesirablepolymerizations is offered by the employment of polymerizationinhibitors. This possibility is used particularly to inhibitpolymerization in process streams comprising various, partly vinylic,monomers, but also DCPD and CPD.

The following classes of inhibitor have been described for this purpose:nitroxides such as TEMPO derivatives, phenyldiamines, hydroxylaminessuch as diethylhydroxylamine (abbreviated as “DEHA”), nitroaromaticssuch as 4,6-dinitro-2-sec-butylphenol (abbreviated as “DNBP”), diphenolssuch as hydroquinone (abbreviated as “HQ”) and p-tert-butylcatechol(abbreviated as “TBC”).

Buccolini et al. (WO2001/047844) describe using a combination ofnitroxide compounds, in particular TEMPO derivatives and aromaticamines, in particular diphenylamines and phenylenediamines, to inhibitpolymerization reactions in hydrocarbon streams. These hydrocarbonstreams comprise butadiene and styrene, but may also comprisecyclopentadiene.

Kazuo et al. (JP62167733A & JP62167734A) describe the admixture ofhydroxylamines and TEMPO derivatives, respectively, to a reactionmixture, for example of cyclopentadiene and dicyclopentadiene with1,3-butadiene, to prevent secondary polymerization reactions.

Ammannati et al. (WO2010/020549) describe a process for producingethylidenenorbornenes wherein dicyclopentadiene is converted intocyclopentadiene in a first step. Various inert solvents can be usedhere, for example diphenyl ether, diphenylmethane, decalin or a mixtureof di- and triaryl ethers. Polymerization inhibitors such as, forexample, 4-oxo-2,2,6,6-tetramethylpiperidine N-oxide (abbreviated as“4-oxo-TEMPO”) and also tert-butylhydroquinone are additionally used.

Cai et al. (Z. Cai, B. Shen, W. Liu, Z. Xin, H. Ling, Energy & Fuels2009, 23, 4077-4081) describe o-nitrophenol, TBC and DEHA as possibleinhibitors. A 3:1 mixture (by weight) of TBC and DEHA is referred to asparticularly advantageous.

Cheng et al. (CN101798255) describe polymerization inhibitors useful forremoving diolefins from the C5 fraction in a cracking process,specifically in the extraction of 1,3-pentadiene. Sodium nitrite, TBC,DEHA and o-nitrophenol are mentioned as possible polymerizationinhibitors.

Ge et al. (CN101104573) describe a process for separating isoprene andcyclopentadiene wherein polymerization inhibitors are employed. Theinhibitors employed can be one or more substances selected fromo-nitrophenol, TBC, DEHA and dihydroxydihydrocinnamic acid.

Chen et aL (CN102060649) describe a process for producingcyclopentadiene wherein HQ, 2,6-dinitrocresol and/or TBC orphenothiazine (abbreviated as “PTZ”) are employed.

Hu et al. (CN1253130) describe a process for removing diolefins from aC5 stream wherein DEHA, TBC or o-nitrophenol can be used as stabilizers.

Lartigue-Peyrou et al. (WO1999/015603) describe mixtures of catecholderivatives and aromatic ethers useful for stabilizing unsaturatedcompounds, including cyclopentadiene and dicyclopentadiene.

Cocuta et al. (RO93489) describe mixtures of sec-butylphenols andphenylenediamines/tert-butylcatechols capable of inhibiting thepolymerization of olefins and diolefins.

However, experiments employing pure cyclopentadiene surprisingly showthat most of the conventionally known inhibitors which have also beenused inter alia for application in the presence of cyclopentadiene havelittle if any efficacy when used to prevent the polymerization ofcyclopentadiene (see Comparative Examples 1 to 6).

This applies for example to the substances described in WO2010/020549,WO2001/047844 and JP62167734, which are efficacious as inhibitors forthe polymerization reactions of monomers such as butadiene or styrene(see Comparative Examples 13 to 16). As shown in Comparative Examples 1to 6, however, these substances fail when used for inhibiting thepolymerization of cyclopentadiene. DNBP, for example, was found to haveno effect with regard to the polymerization of cyclopentadiene. TheTEMPO derivatives likewise have only minimal efficacy.

It was accordingly surprisingly found that conventional inhibitors arenot very suitable for preventing the polymerization in CPD orDCPD-containing process streams and are mainly or even exclusivelyeffective as inhibitors of free-radical polymerizations observed withvinyl-containing monomers. The fact that certain substances have beendescribed as generally useful for stabilizing olefinically unsaturatedmonomers cannot be used, therefore to infer their usefulness forinhibiting the polymerization of cyclopentadiene, dicyclopentadiene orother cyclopentadiene compounds.

It is an object of the present invention to provide a polymerizationinhibitor having good activity against undesired polymerizations ofdicyclopentadiene and cyclopentadiene. The inhibitor should also work athigh to very high temperatures and exhibit an improved performance overthe inhibitors previously conventionally employed.

SUMMARY OF THE INVENTION

It has now been found that this object is achieved, utterlysurprisingly, by the employment of certain quinone methides [hereinbelow“Compound of structure (I)” ]. They have outstanding inhibitoryproperties with regard to the polymerization of CPD and DCPD, which issurprising in itself because they have hitherto merely been describedfor inhibiting the polymerization of vinylic monomers (EP2055691A1;EP0737659A1; EP0737660A1, WO2012/173909). It is further surprising thatthe inhibitory effect of the quinone methides according to the inventionis superior to that of the conventionally known CPD and DCPDpolymerization inhibitors. The quinone methides according to theinvention may be used neat as well as in diluted form. Thecyclopentadiene compound can be in pure form or as a component in aprocess stream.

The invention relates to a composition (AB) comprising (A) and (B), aprocess for inhibiting the polymerization of (B) which is characterizedin that (A) and (B) are brought into contact, and also to the use of (A)for inhibiting the polymerization of (B),

wherein

(A) is at least one compound of structure (I)

where

-   -   R¹ and R² are each independently hydrogen, alkyl of 1 to 18        carbon atoms, cycloalkyl of 3 to 15 carbon atoms, aryl of 6 to        15 carbon atoms or phenylalkyl of 7 to 15 carbon atoms;    -   R³ is —CN, —COOH, —COOR⁴, —COR⁵, —OCOR⁶, —CONR⁷R⁸, —PO(OR⁹)₂,        —O—R¹⁰, —S—R¹¹, —R¹², —C≡C—R¹³ or halogen; where        -   R⁴, R⁵, R⁶ are alkyl of 1 to 18 carbon atoms, cycloalkyl of            3 to 12 carbon atoms, aryl of 6 to 10 carbon atoms;        -   R⁷ and R⁸ are each independently hydrogen; alkyl of 1 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            alkylamino having 1 to 4 carbon atoms, dialkylamino having 2            to 8 carbon atoms and hydroxyl; phenylalkyl of 7 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            hydroxyl, alkyl having 1 to 4 carbon atoms, alkylamino            having 1 to 4 carbon atoms and dialkylamino having 2 to 8            carbon atoms; aryl of 6 to 10 carbon atoms which is            unsubstituted or substituted with at least one substituent            selected from the group consisting of alkyl having 1 to 4            carbon atoms, alkylamino having 1 to 4 carbon atoms,            dialkylamino having 2 to 8 carbon atoms and hydroxyl; or            NR⁷R⁸ is morpholino, piperidino or pyrrolidino;        -   R⁹, R¹⁰, R¹¹ are hydrogen; alkyl of 1 to 15 carbon atoms            which is unsubstituted or substituted with at least one            substituent selected from the group consisting of hydroxyl,            dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3,            4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and where R¹⁴            is alkyl of 1 to 6 carbon atoms;        -   cycloalkyl of 3 to 15 carbon atoms which is unsubstituted or            substituted with at least one substituent selected from the            group consisting of hydroxyl, dialkylamino, —OR¹⁴,            —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3, 4, 5, 6, 7, 8, 9 or            10 and y is 1, 2, 3 or 4 and where R¹⁴ is alkyl of 1 to 6            carbon atoms; phenylalkyl of 7 to 15 carbon atoms which is            unsubstituted or substituted with at least one substituent            selected from the group consisting of hydroxyl,            dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3,            4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and where R¹⁴            is alkyl of 1 to 6 carbon atoms; or aryl of 6 to 15 carbon            atoms which is unsubstituted or substituted with at least            one substituent selected from the group consisting of            hydroxyl, dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x            is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and            where R¹⁴ is alkyl of 1 to 6 carbon atoms;            -   R¹² is 2-pyridyl, 3-pyridyl, 4-pyridyl, 2-thienyl,                3-thienyl, 2-pyrryl, 3-pyrryl, 2-furyl, 3-furyl or aryl                of 6 to 15 carbon atoms; wherein the radicals 2-pyridyl,                3-pyridyl, 4-pyridyl, 2-thienyl, 3-thienyl, 2-pyrryl,                3-pyrryl, 2-furyl, 3-furyl or aryl of 6 to 15 carbon                atoms are unsubstituted or substituted with at least one                substituent selected from the group consisting of                hydroxyl, nitro, amino, cyano, carboxyl, aminocarbonyl,                halogen, alkyl of 1 to 8 carbon atoms, alkoxy of 1 to 8                carbon atoms, alkylthio of 1 to 8 carbon atoms,                alkylamino of 1 to 8 carbon atoms, dialkylamino of 2 to                8 carbon atoms and a carboxylic ester group of 2 to 8                carbon atoms;            -   R¹³ is hydrogen, alkyl of 1 to 12 carbon atoms, aryl of                6 to 10 carbon atoms, wherein the aryl of 6 to 10 carbon                atoms is unsubstituted or substituted with at least one                substituent selected from the group consisting of                hydroxyl, nitro, amino, cyano, carboxyl, aminocarbonyl,                halogen, alkyl of 1 to 8 carbon atoms, alkoxy of 1 to 8                carbon atoms, alkylthio of 1 to 8 carbon atoms,                alkylamino of 1 to 8 carbon atoms, dialkylamino of 2 to                8 carbon atoms and a carboxylic ester group of 2 to 8                carbon atoms;

wherein the substituents R¹, R² and R³ are the same or different; and(B) is at least one cyclopentadiene compound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the results obtained in Examples 1 to 11 on comparingcertain compounds previously employed and the compounds of the inventionin a test of their ability to stabilize pure (di-)cyclopentadieneagainst polymerization at 170° C. oil bath temperature. The x-axisrepresents the time in minutes and the y-axis the peak area measuredusing ELS. Abbreviations: 4-hydroxy-TEMPO (4-HT), 4-butoxy-TEMPO (4-BT),tert-butyl-catechol (TBC) and dinitro-sec-butylphenol (DNBP). Thestructure of QM-1 is (V), the structure of QM-2 is (VI), the structureof QM-5 is (X), the structure of QM-7 is (XII) and the structure ofQM-11 is (XVI).

FIG. 2 shows some results obtained in Examples 12 to 21 on comparingcertain compounds previously employed and the compounds of the inventionin a test of their ability to stabilize styrene against polymerizationat 110° C. The x-axis represents the time in minutes, the y-axis thepolymer content in %. Further results are found in Table 2.Abbreviations: 4-hydroxy-TEMPO (4-HT), tert-butyl-catechol (TBC) anddinitro-sec-butylphenol (DNBP). The structure of QM-1 is (V), thestructure of QM-5 is (X).

Exact experimental descriptions are found in the Examples section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS General Terms

Throughout this description all ranges described include all values andsub-ranges therein, unless otherwise specified.

Additionally, the indefinite article “a” or “an” carries the meaning of“one or more” throughout the description, unless otherwise specified.

The term “cyclopentadiene compound” for the purposes of the presentinvention describes a compound selected from the group consisting ofcyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

Cyclopentadiene (CPD) has structure (II).

Dicyclopentadiene (DCPD) has structure (111) and possesses two isomericforms, endo-dicyclopentadiene (endo-DCPD) and exo-dicyclopentadiene(exo-DCPD).

The term “(di)cyclopentadiene” for the purposes of the present inventionrefers to mixtures of CPD and DCPD.

The expression “polymerization of (B)” describes any polymerizationinvolving (B), preferably an oligomerization/polymerization with itselfor vinylic structures.

The term “alkylated cyclopentadiene” for the purposes of the presentinvention describes compounds of structure (IV).

where, in the compound of structure (IV), at least one of R¹⁵, R¹⁶, R¹⁷,R¹⁸, R¹⁹, R²⁰ is alkyl having 1 to 18 carbon atoms while the others areeach hydrogen. In one preferred embodiment, “alkylated cyclopentadiene”is monoalkylcyclopentadiene or dialkylcyclopentadiene, more preferablymonoalkylcyclopentadiene.

The term “monoalkylcyclopentadiene” for the purposes of the presentinvention describes the compounds of structure (IV) where precisely oneof R¹⁵, R¹⁶, R¹⁷, R¹⁸, R¹⁹, R²⁰ is alkyl having 1 to 18 carbon atoms,preferably alkyl having 1 to 6 carbon atoms, more preferably methyl orethyl, most preferably methyl, while the others are each hydrogen.

The term “dialkylcyclopentadiene” for the purposes of the presentinvention describes the compounds of structure (IV) where precisely twoof R¹⁵, R¹⁶, R¹⁷, R¹⁸, R¹⁹, R²⁰ are each independently alkyl of 1 to 18carbon atoms, preferably alkyl of 1 to 6 carbon atoms, more preferablymethyl or ethyl, most preferably methyl, while the others are eachhydrogen.

The term “alkylated dicyclopentadiene” for the purposes of the presentinvention describes any molecule of structure (III) where at least onehydrogen is replaced by alkyl of 1 to 18 carbon atoms, preferably byalkyl of 1 to 6 carbon atoms, more preferably by methyl or ethyl.

The term “alkylated dicyclopentadiene” for the purposes of the presentinvention describes in one very particularly preferred embodiment anymolecule of structure (III) where precisely one hydrogen, precisely twohydrogens, precisely three hydrogens or precisely four hydrogens is/arereplaced by alkyl of 1 to 6 carbon atoms, more preferably by methyl orethyl.

Alkyl of 1 to 18 carbon atoms has for the purposes of the presentinvention between 1 and 18 saturated carbon atoms and may be linear orbranched and may be more particularly selected from methyl, ethyl,n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl,n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl,1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl,1-ethylpropyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl,4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl,2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl,2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl,1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl, heptyl, octyl, nonyl,decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl,heptadecyl, octadecyl. Alkyl of 1 to 12 carbon atoms has for thepurposes of the present invention between 1 and 12 saturated carbonatoms and may be linear or branched and may be more particularlyselected from methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl,iso-butyl, tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl,2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl,1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl,2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl,1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl,1-ethyl-2-methylpropyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl.

Alkyl of 1 to 6 carbon atoms has for the purposes of the presentinvention between 1 and 6 saturated carbon atoms and may be linear orbranched and may be more particularly selected from methyl, ethyl,n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl,n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl,1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl,1-ethylpropyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl,4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl,2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl,2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl,1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl.

Alkyl of 1 to 4 carbon atoms has for the purposes of the presentinvention between 1 and 4 saturated carbon atoms and may be linear orbranched and may be more particularly selected from methyl, ethyl,n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl.

Alkyl of 1 to 3 carbon atoms has for the purposes of the presentinvention between 1 and 3 saturated carbon atoms and may be linear orbranched and may be more particularly selected from methyl, ethyl,n-propyl, iso-propyl.

Cycloalkyl of 3 to 15 carbon atoms is for the purposes of the presentinvention more particularly selected from cyclopropyl, cyclobutyl,cyclopropylmethyl, cyclopentyl, cyclobutylmethyl, cyclohexyl,cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl, cycloundecyl,cyclododecyl, cyclotridecyl, cyclotetradecyl, cyclopentadecyl.

Cycloalkyl of 3 to 12 carbon atoms is for the purposes of the presentinvention more particularly selected from cyclopropyl, cyclobutyl,cyclopropylmethyl, cyclopentyl, cyclobutylmethyl, cyclohexyl,cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl, cycloundecyl,cyclododecyl.

Aryl of 6 to 15 carbon atoms is more particularly selected from phenyl,1-naphthyl, 2-naphthyl, 9-anthryl, 9-phenanthryl.

Aryl of 6 to 10 carbon atoms is for the purposes of the presentinvention more particularly selected from phenyl, 1-naphthyl,2-naphthyi.

Phenylalkyl of 7 to 15 carbon atoms comprises for the purposes of thepresent invention branched or unbranched alkyl with an attached phenylring, and is more particularly selected from benzyl, phenylethyl,α-methylbenzyl, 3-phenylpropyl, phenyl-2-methylethyl,phenyl-1-methylethyl, α,α-dimethylbenzyl, butylphenyl, hexylphenyl,octylphenyl, nonylphenyl, preferably benzyl.

Alkylamino of 1 to 4 carbon atoms for the purposes of the presentinvention refers more particularly to an amino moiety comprising analkyl group of 1 to 4 carbon atoms, and is preferably selected frommethylamino, ethylamino, propylamino, isopropylamino and butylamino.

Dialkylamino for the purposes of the present invention is moreparticularly an amino moiety which bears two alkyl groups, and is moreparticularly dialkylamino of 2 to 8 carbon atoms.

Dialkylamino of 2 to 8 carbon atoms for the purposes of the presentinvention refers more particularly to an amino moiety comprising twoalkyl groups of 1 to 4 carbon atoms, wherein these alkyl groups of 1 to4 carbon atoms can be the same or different, and is preferably selectedfrom dimethylamino, diethylamino, dipropylamino, dibutylamino,methylethylamino and methylbutylamino.

“High-boiling hydrocarbon cuts” for the purposes of the presentinvention denotes aliphatic or aromatic hydrocarbon cuts having fixedboiling ranges, in particular aromatic hydrocarbons having a boilingpoint (at atmospheric pressure) in the range from 155° C. to 300° C.,these preferably contain one or more substances selected from the groupconsisting of n-propylbenzene, 1-methyl-4-ethylbenzene,1-methyl-3-ethylbenzene, mesitylene, 1-methyl-2-ethylbenzene,1,2,4-trimethylbenzene, 1,2,3-trimethylbenzene, indane,1,3-diethylbenzene, 1-methyl-4-propylbenzene, 1-methyl-3-propylbenzene,1,2,4,5-tetramethylbenzene, 1,2,3,5-tetramethylbenzene and naphthalene.

Process According to the Invention

The expression “process according to the invention” is synonymous with“process for inhibiting the polymerization of (B)”.

The invention provides a process for inhibiting the polymerization of(B), said process being characterized in that (A) and (B) are broughtinto contact, wherein

(A) is at least one compound of structure (I)

wherein

-   -   R¹ and R² are each independently hydrogen, alkyl of 1 to 18        carbon atoms, cycloalkyl of 3 to 15 carbon atoms, aryl of 6 to        15 carbon atoms or phenylalkyl of 7 to 15 carbon atoms;    -   R³ is —CN, —COOH, —COOR⁴, —COR⁵, —OCOR⁶, —CONR⁷R8, —PO(OR⁹)₂,        —O—R¹⁰, —S—R¹¹, —R¹², —C≡C—R¹³ or halogen;    -   wherein        -   R⁴, R⁵, R⁶ is alkyl of 1 to 18 carbon atoms, cycloalkyl of 3            to 12 carbon atoms, aryl of 6 to 10 carbon atoms;        -   R⁷ and R⁸ are each independently hydrogen; alkyl of 1 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            alkylamino having 1 to 4 carbon atoms, dialkylamino having 2            to 8 carbon atoms and hydroxyl; phenylalkyl of 7 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            hydroxyl, alkyl having 1 to 4 carbon atoms, alkylamino            having 1 to 4 carbon atoms and dialkylamino having 2 to 8            carbon atoms; aryl of 6 to 10 carbon atoms which is            unsubstituted or substituted with at least one substituent            selected from the group consisting of alkyl having 1 to 4            carbon atoms, alkylamino having 1 to 4 carbon atoms,            dialkylamino having 2 to 8 carbon atoms and hydroxyl; or            NR⁷R⁸ is morpholino, piperidino or pyrrolidino;        -   R⁹, R¹⁰, R¹¹ are hydrogen; alkyl of 1 to 15 carbon atoms            which is unsubstituted or substituted with at least one            substituent selected from the group consisting of hydroxyl,            dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3,            4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and where R¹⁴            is alkyl of 1 to 6 carbon atoms; cycloalkyl of 3 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            hydroxyl, dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x            is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and            where R¹⁴ is alkyl of 1 to 6 carbon atoms; phenylalkyl of 7            to 15 carbon atoms which is unsubstituted or substituted            with at least one substituent selected from the group            consisting of hydroxyl, dialkylamino, —OR¹⁴,            —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3, 4, 5, 6, 7, 8, 9 or            10 and y is 1, 2, 3 or 4 and where R¹⁴ is alkyl of 1 to 6            carbon atoms; or aryl of 6 to 15 carbon atoms which is            unsubstituted or substituted with at least one substituent            selected from the group consisting of hydroxyl,            dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3,            4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and where R¹⁴            is alkyl of 1 to 6 carbon atoms;            -   R¹² is 2-pyridyl, 3-pyridyl, 4-pyridyl, 2-thienyl,                3-thienyl, 2-pyrryl, 3-pyrryl, 2-furyl, 3-furyl or aryl                of 6 to 15 carbon atoms; wherein the radicals 2-pyridyl,                3-pyridyl, 4-pyridyl, 2-thienyl, 3-thienyl, 2-pyrryl,                3-pyrryl, 2-furyl, 3-furyl or aryl of 6 to 15 carbon                atoms are unsubstituted or substituted with at least one                substituent selected from the group consisting of                hydroxyl, nitro, amino, cyano, carboxyl, aminocarbonyl,                halogen, alkyl of 1 to 8 carbon atoms, alkoxy of 1 to 8                carbon atoms, alkyithio of 1 to 8 carbon atoms,                alkylamino of 1 to 8 carbon atoms, dialkylamino of 2 to                8 carbon atoms and a carboxylic ester group of 2 to 8                carbon atoms;            -   R¹³ is hydrogen, alkyl of 1 to 12 carbon atoms, aryl of                6 to 10 carbon atoms, wherein the aryl of 6 to 10 carbon                atoms is unsubstituted or substituted with at least one                substituent selected from the group consisting of                hydroxyl, nitro, amino, cyano, carboxyl, aminocarbonyl,                halogen, alkyl of 1 to 8 carbon atoms, alkoxy of 1 to 8                carbon atoms, alkylthio of 1 to 8 carbon atoms,                alkylamino of 1 to 8 carbon atoms, dialkylamino of 2 to                8 carbon atoms and a carboxylic ester group of 2 to 8                carbon atoms;    -   wherein the substituents R¹, R² and R³ are the same or        different; and (B) is at least one cyclopentadiene compound.

In one preferred embodiment of the process according to the invention,R¹ and R² in the compound of structure (I) are each independentlyselected from the group consisting of methyl, ethyl, n-propyl,iso-propyl, n-butyl, iso-butyl and tert-butyl; and R³═—CN, —COOH,—COOR⁴, —COR⁵, —OCOR⁶, —CONR⁷R⁸, —PO(OR⁹)₂, —O—R¹⁰, —S—R¹¹, —R¹²—C≡C—R¹³ or halogen; wherein R⁴, R⁵, R⁶ are alkyl of 1 to 8 carbon atomsor phenyl;

-   -   R⁷R⁸ are each independently hydrogen or alkyl of 1 to 4 carbon        atoms, or NR⁷R⁸ is morpholino or piperidino;    -   R⁹, R¹⁰, R¹¹ are alkyl of 1 to 8 carbon atoms or phenyl;    -   R¹² is 2-furyl, 3-furyl or aryl of 6 to 15 carbon atoms, wherein        the radicals 2-furyl, 3-furyl or aryl of 6 to 15 carbon atoms        are unsubstituted or substituted with at least one substituent        selected from the group consisting of hydroxyl and alkyl of 1 to        8 carbon atoms;    -   R¹³ is hydrogen, alkyl of 1 to 12 carbon atoms, aryl of 6 to 10        carbon atoms.

In one more preferred embodiment of the process according to theinvention, R¹ and R² are each independently selected from the groupconsisting of methyl and tert-butyl in the compound of structure (I);and R³ is —CN, —COOH, —COOR⁴, —O—R¹⁰, —S—R¹¹, —R¹², —C≡C—R¹³ or halogen;wherein

-   -   R⁴ is alkyl of 1 to 4 carbon atoms;    -   R¹⁰, R¹¹ are alkyl of 1 to 6 carbon atoms;    -   R¹² is 2-furyl, 3-furyl or aryl of 6 to 12 carbon atoms, wherein        the radicals 2-furyl, 3-furyl or aryl of 6 to 12 carbon atoms        are unsubstituted or substituted with at least one substituent        selected from the group consisting of hydroxyl and alkyl of 1 to        8 carbon atoms;    -   R¹³ is aryl of 6 to 10 carbon atoms.

In one still more preferred embodiment of the process according to theinvention R¹ and R² are each tert-butyl in the compound of structure(I); and

R³ is —CN, —COOH, —O—R¹⁰, —S—R¹¹, —R¹² or —C≡C—R¹³;

-   -   wherein    -   R¹⁰ is methyl, ethyl, iso-propyl, n-propyl, n-butyl, iso-butyl,        sec-butyl, tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,        3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,        2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl,        2-methylpentyl, 3-methylpentyl, 4-methylpentyl,        1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl,        2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl,        1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl,        1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl or        1-ethyl-2-methylpropyl, preferably methyl, ethyl, iso-propyl,        n-propyl, sec-butyl, n-butyl, n-pentyl, or n-hexyl;    -   R¹¹ is alkyl of 1 to 6 carbon atoms;    -   R¹² is 2-furyl, 3-furyl or phenyl, wherein phenyl is        unsubstituted or substituted with at least one substituent        selected from the group consisting of hydroxyl and alkyl of 1 to        8 carbon atoms;    -   R¹³ is phenyl.

In a first very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is CN, the compound of structure (I) then havingstructure (V) (hereinafter also abbreviated as “QM-1”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a second very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is phenyl, the compound of structure (I) thenhaving structure (VI) (hereinafter also abbreviated as “QM-2”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a third very particularly preferred embodiment of the processaccording to the invention, the compound of structure (1) has R¹ and R²being tert-butyl and R³ is 3,5-di-tert-butyl-4-hydroxyphenyl, thecompound of structure (I) then having structure (VII) (hereinafter alsoabbreviated as “QM-3”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fourth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is 2-furyl, the compound of structure (I) thenhaving structure (VIII) (hereinafter also abbreviated as “QM-4”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fifth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —OR¹⁰,

whereinR¹⁰ is methyl, ethyl, iso-propyl, n-propyl, n-butyl, sec-butyl,iso-butyl, tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl,2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl,1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl,2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl,1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl or1-ethyl-2-methylpropyl, preferably methyl, ethyl, iso-propyl, n-propyl,n-butyl, sec-butyl, tert-butyl, n-pentyl or n-hexyl, more preferablymethyl, ethyl, n-propyl, iso-propyl, n-butyl, n-pentyl or n-hexyl; thecompound of structure (I) then having structure (IX)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a sixth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —OCH₃, the compound of structure (I) thenhaving structure (X) (hereinafter also abbreviated as “QM-5”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a seventh very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —OCH₂CH₃, the compound of structure (I) thenhaving structure (XI) (hereinafter also abbreviated as “QM-6”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In an eighth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —OCH₂CH₂CH₃, the compound of structure (I)then having structure (XII) (hereinafter also abbreviated as “QM-7”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a ninth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —OCH(CH₃)₂, the compound of structure (I)then having structure (XIII) (hereinafter also abbreviated as “QM-8”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a tenth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —OCH₂CH₂CH₂CH₃, the compound of structure (I)then having structure (XIV) (hereinafter also abbreviated as “QM-9”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In an eleventh very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —OCH₂CH₂CH₂CH₂CH₃, the compound of structure(I) then having structure (XV) (hereinafter also abbreviated as “QM-10”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a twelfth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —OCH₂CH₂CH₂CH₂CH₂CH₃, the compound ofstructure (I) then having structure (XVI) (hereinafter abbreviated as“QM-11”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a thirteenth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —C≡C-phenyl, the compound of structure (I)then having structure (XVII) (hereinafter abbreviated as “QM-12”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fourteenth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —COOH, the compound of structure (I) thenhaving structure (XVIII) (hereinafter abbreviated as “QM-13”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fifteenth very particularly preferred embodiment of the processaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —SCH₂CH₂CH₂CH₃, the compound of structure (I)then having structure (XIX) (hereinafter abbreviated as “QM-14”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

(A) may be used in the process according to the invention in gaseousform, as a solid material (as a powder, for example) or as a liquid, inparticular as a solid material (as a powder, for example) or as aliquid, preferably as a liquid. (A) used as a liquid in the processaccording to the invention is more particularly used as a melt or in theform of a solution in (C), where “(C)” has the meaning “at least onesolvent”.

Any material may be useful as a solvent in the process according to theinvention provided (A) is soluble therein in the desired concentrationrange and it is both compatible with (A) and does not have a disruptiveeffect on the process according to the invention, and may be an apolarsolvent, preferably an apolar aromatic or aliphatic solvent. It may bemore preferable for the solvent in the process according to theinvention to be selected from the group consisting of benzene, mono- orpolyalkylated aromatics, alkanes having a carbon number of 6 to 15,cycloalkanes having a carbon number of 6 to 15, high-boiling hydrocarboncuts, ethers having a carbon number of 6 to 15 and esters having acarbon number of 6 to 15. It may be still more preferable for thesolvent in the process according to the invention to be selected fromthe group consisting of benzene, toluene, ethylbenzene, xylene, styreneand high-boiling aromatic hydrocarbon cuts. It may be particularlypreferable for the solvent in the process according to the invention tobe selected from the group consisting of toluene, ethylbenzene, xyleneand styrene. Alternatively, the cyclopentadiene compound itself can alsoserve as the solvent in the process according to the invention.

When (A) is used in the process according to the invention in the formof a solution in (C), the total weight of all compounds of structure (I)in said solution (AC) preferably has an (m/m) ratio to the total weightof all solvents in said solution (AC) in the range from 1:1000 to 100:1,more preferably in the range from 1:100 to 10:1 and still morepreferably in the range from 1:10 to 3:1.

(B) may be present in the process according to the invention in gaseousform, as a liquid or as a solid material, in particular in gaseous formor as a liquid, preferably as a liquid. (B) as a liquid is still morepreferably in the form of a melt or solution. It may be particularlypreferable for (B) to be in the form of a solution. Such a solution in afirst very particularly preferred embodiment is a process stream asobtained in cracking processes. (B) may typically be present in such aprocess stream at from 0.0001 wt % to 15 wt %.

In an alternative second very particularly preferred embodiment, thesolution may be a process stream as generated in the production of DCPDand/or CPD itself. (B) is typically present in such a process stream atbetween 15 and 100 wt %, preferably between 70 and 100 wt % and stillmore preferably between 70 and 99.99 wt %.

The expression “bringing (A) and (B) into contact” for the purposes ofthe invention is to be understood as meaning in particular that (A) isadmixed to (B) or (B) is admixed to (A). Admixing (A) to (B) or (B) to(A) can be effected according to conventionally known methods.

(A) may be admixed with advantage in the process according to theinvention into any feedstream or outflow line of a distillation column,into the in- and outflow line of a heat exchanger or into the in- andoutflow line of a vaporizer (“reboiler”) or into the in- and outflowline of a condenser or into the in- and outflow line of a reactor. (A)may also be added in the process according to the invention to storagetanks containing a process stream comprising (B). (A) may be admixed to(B) not only before but also during a process, for example a productionor purification process.

An effective amount of (A) is admixed in the process according to theinvention. The term “effective amount of (A)” in the context of thisinvention may be understood as meaning the amount of (A) needed todelay/prevent the undesired polymerization of (B). This effective amountdepends on the conditions under which the cyclopentadiene compound, ormixture of two or more cyclopentadiene compounds, is stored or handledand can readily be determined from case to case by a person skilled inthe art. For example, the cracking of dicyclopentadiene requires byreason of the higher temperatures a higher amount of (A) than thestoring of (B) at for instance room temperature.

(A) may preferably be used in the process according to the invention insuch an amount that the total concentration of all compounds ofstructure (I) is between 10 ppb on a mass to mass basis (m/m) and100,000 ppm (m/m), more preferably between 1 ppm (m/m) and 50,000 ppm(m/m), even more preferably between 10 ppm and 10,000 ppm (m/m), mostpreferably between 100 ppm and 5000 ppm (m/m), each based on the totalweight of all cyclopentadiene compounds.

The temperature at which the process according to the invention may becarried out is not subject to any in-principle limitation; on thecontrary, it is a feature of the present invention that the processaccording to the invention can be carried out not only at low but alsoat high temperatures, in particular in the range from 0° C. to 250° C.,preferably 0° C. to 200° C.

The process according to the invention may utilize a polymerizationinhibitor (D) as well as (A). Polymerization inhibitors of this type areconventionally known, examples being nitroxides such as, for instance,oxo-TEMPO or 4-hydroxy-TEMPO, phenylenediamines, hydroxylamines such asdiethylhydroxylamine (DEHA), nitro- or nitrosoaromatics such as DNBP,(di)phenols such as hydroquinone, TBC or 2,6-di-tert-butylphenol,benzoquinones, phenothiazines such as PTZ.

Composition According to the Invention

The invention also provides a composition (AB), comprising (A) and (B),wherein(A) is at least one compound of structure (I)

wherein

-   -   R¹ and R² are each independently hydrogen, alkyl of 1 to 18        carbon atoms, cycloalkyl of 3 to 15 carbon atoms, aryl of 6 to        15 carbon atoms or phenylalkyl of 7 to 15 carbon atoms;    -   R³ is —CN, —COOH, —COOR⁴, —COR⁵, —OCOR⁶, —CONR⁷R⁸, —PO(OR⁹)₂,        —O—R¹⁰, —S—R¹¹, —R¹², —C≡C—R¹³ or halogen;    -   wherein        -   R⁴, R⁵, R⁶ are alkyl of 1 to 18 carbon atoms, cycloalkyl of            3 to 12 carbon atoms, aryl of 6 to 10 carbon atoms;        -   R⁷ and R⁸ are each independently hydrogen; alkyl of 1 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            alkylamino having 1 to 4 carbon atoms, dialkylamino having 2            to 8 carbon atoms and hydroxyl; phenylalkyl of 7 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            hydroxyl, alkyl having 1 to 4 carbon atoms, alkylamino            having 1 to 4 carbon atoms and dialkylamino having 2 to 8            carbon atoms; aryl of 6 to 10 carbon atoms which is            unsubstituted or substituted with at least one substituent            selected from the group consisting of alkyl having 1 to 4            carbon atoms, alkylamino having 1 to 4 carbon atoms,            dialkylamino having 2 to 8 carbon atoms and hydroxyl; or            NR⁷R⁸ is morpholino, piperidino or pyrrolidino;        -   R⁹, R¹⁰, R¹¹ are hydrogen; alkyl of 1 to 15 carbon atoms            which is unsubstituted or substituted with at least one            substituent selected from the group consisting of hydroxyl,            dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3,            4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and where R¹⁴            is alkyl of 1 to 6 carbon atoms;        -   cycloalkyl of 3 to 15 carbon atoms which is unsubstituted or            substituted with at least one substituent selected from the            group consisting of hydroxyl, dialkylamino, —OR¹⁴,            —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3, 4, 5, 6, 7, 8, 9 or            10 and y is 1, 2, 3 or 4 and where R¹⁴ is alkyl of 1 to 6            carbon atoms; phenylalkyl of 7 to 15 carbon atoms which is            unsubstituted or substituted with at least one substituent            selected from the group consisting of hydroxyl,            dialkylamino, —OR⁴, —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3,            4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and where R¹⁴            is alkyl of 1 to 6 carbon atoms; or aryl of 6 to 15 carbon            atoms which is unsubstituted or substituted with at least            one substituent selected from the group consisting of            hydroxyl, dialkylamino, —OR¹⁴, —[O(CH₂)_(y)])_(x)H, where x            is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and            where R¹⁴ is alkyl of 1 to 6 carbon atoms; R¹² is 2-pyridyl,            3-pyridyl, 4-pyridyl, 2-thienyl, 3-thienyl, 2-pyrryl,            3-pyrryl, 2-furyl, 3-furyl or aryl of 6 to 15 carbon atoms;            wherein the radicals 2-pyridyl, 3-pyridyl, 4-pyridyl,            2-thienyl, 3-thienyl, 2-pyrryl, 3-pyrryl, 2-furyl, 3-furyl            or aryl of 6 to 15 carbon atoms are unsubstituted or            substituted with at least one substituent selected from the            group consisting of hydroxyl, nitro, amino, cyano, carboxyl,            aminocarbonyl, halogen, alkyl of 1 to 8 carbon atoms, alkoxy            of 1 to 8 carbon atoms, alkylthio of 1 to 8 carbon atoms,            alkylamino of 1 to 8 carbon atoms, dialkylamino of 2 to 8            carbon atoms and a carboxylic ester group of 2 to 8 carbon            atoms;            -   R¹³ is hydrogen, alkyl of 1 to 12 carbon atoms, aryl of                6 to 10 carbon atoms, wherein the aryl of 6 to 10 carbon                atoms is unsubstituted or substituted with at least one                substituent selected from the group consisting of                hydroxyl, nitro, amino, cyano, carboxyl, aminocarbonyl,                halogen, alkyl of 1 to 8 carbon atoms, alkoxy of 1 to 8                carbon atoms, alkylthio of 1 to 8 carbon atoms,                alkylamino of 1 to 8 carbon atoms, dialkylamino of 2 to                8 carbon atoms and a carboxylic ester group of 2 to 8                carbon atoms;

wherein the substituents R¹, R² and R³ are the same or different, and(B) is at least one cyclopentadiene compound.

In one preferred embodiment of the composition (AB) according to theinvention, R¹ and R² in the compound of structure (I) are eachindependently selected from the group consisting of methyl, ethyl,n-propyl, iso-propyl, n-butyl, iso-butyl and tert-butyl; and

R³ is —CN, —COOH, —COOR⁴, —COR⁵, —OCOR⁶, —CONR⁷R⁸, —PO(OR⁹)₂, —O—R¹⁰,—S—R¹¹, —R¹² —C≡C—R¹³ or halogen;

-   -   wherein    -   R⁴, R⁵, R⁶ are alkyl of 1 to 8 carbon atoms or phenyl;    -   R⁷R⁸ are each independently hydrogen or alkyl of 1 to 4 carbon        atoms, or NR⁷R⁸ is morpholino or piperidino;    -   R⁹, R¹⁰, R¹¹ are alkyl of 1 to 8 carbon atoms or phenyl;    -   R¹² is 2-furyl, 3-furyl or aryl of 6 to 15 carbon atoms, wherein        the radicals 2-furyl, 3-furyl or aryl of 6 to 15 carbon atoms        are unsubstituted or substituted with at least one substituent        selected from the group consisting of hydroxyl and alkyl of 1 to        8 carbon atoms; and    -   R¹³ is hydrogen, alkyl of 1 to 12 carbon atoms, aryl of 6 to 10        carbon atoms.

In one more preferred embodiment of the composition (AB) according tothe invention, R¹ and R² in the compound of structure (I) are eachindependently selected from the group consisting of methyl andtert-butyl; and

-   -   R³ is —CN, —COOH, —COOR⁴, —O—R10, —S—R¹¹, —R¹², —C≡C—R¹³ or        halogen;    -   wherein    -   R⁴ is alkyl of 1 to 4 carbon atoms;    -   R¹⁰, R¹¹ are alkyl of 1 to 6 carbon atoms;    -   R¹² is 2-furyl, 3-furyl or aryl of 6 to 12 carbon atoms, wherein        the radicals 2-furyl, 3-furyl or aryl of 6 to 12 carbon atoms        are unsubstituted or substituted with at least one substituent        selected from the group consisting of hydroxyl and alkyl of 1 to        8 carbon atoms;    -   R¹³ is aryl of 6 to 10 carbon atoms.

In one still more preferred embodiment of the composition (AB) accordingto the invention, R¹ and R² are each tert-butyl in the compound ofstructure (I); and

R³ is —CN, —COOH, —O—R¹⁰, —S—R¹¹, —R¹² or —C≡C—R¹³;

-   -   wherein    -   R¹⁰ is methyl, ethyl, iso-propyl, n-propyl, n-butyl, iso-butyl,        sec-butyl, tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,        3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,        2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl,        2-methylpentyl, 3-methylpentyl, 4-methylpentyl,        1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl,        2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl,        1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl,        1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl or        1-ethyl-2-methylpropyl, preferably methyl, ethyl, iso-propyl,        n-propyl, sec-butyl, n-butyl, n-pentyl or n-hexyl;    -   R¹¹ is alkyl of 1 to 6 carbon atoms;    -   R¹² is 2-furyl, 3-furyl or phenyl, wherein phenyl is        unsubstituted or substituted with at least one substituent        selected from the group consisting of hydroxyl and alkyl of 1 to        8 carbon atoms; and    -   R¹³ is phenyl.

In a first very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is CN, the compound of structure (I) thenhaving structure (V) (hereinafter also abbreviated as “QM-1”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a second very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is phenyl, the compound of structure (I)then having structure (VI) (hereinafter also abbreviated as “QM-2”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a third very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is 3,5-di-tert-butyl-4-hydroxyphenyl, thecompound of structure (I) then having structure (VII) (hereinafter alsoabbreviated as “QM-3”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fourth very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is 2-furyl, the compound of structure (I)then having structure (VIII) (hereinafter also abbreviated as “QM-4”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fifth very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is —OR¹⁰, wherein R¹⁰ is methyl, ethyl,iso-propyl, n-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl,n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl,1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl,1-ethylpropyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl,4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl,2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl,2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl,1-ethyl-1-methylpropyl or 1-ethyl-2-methylpropyl, preferably methyl,ethyl, iso-propyl, n-propyl, n-butyl, sec-butyl, tert-butyl, n-pentyl orn-hexyl, more preferably methyl, ethyl, n-propyl, iso-propyl, n-butyl,n-pentyl or n-hexyl;

the compound of structure (I) then having structure (IX)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a sixth very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (i) has R¹and R² being tert-butyl and R³ is —OCH₃, the compound of structure (I)then having structure (X) (hereinafter also abbreviated as “QM-5”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a seventh very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (1) has R¹and R² being tert-butyl and R³ is —OCH₂CH₃, the compound of structure(I) then having structure (XI) (hereinafter also abbreviated as “QM-6”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In an eighth very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is —OCH₂CH₂CH₃, the compound of structure(I) then having structure (XII) (hereinafter also abbreviated as “QM-7”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a ninth very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is —OCH(CH₃)₂, the compound of structure(I) then having structure (XIII) (hereinafter also abbreviated as“QM-8”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a tenth very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is —OCH₂CH₂CH₂CH₃, the compound ofstructure (I) then having structure (XIV) (hereinafter also abbreviatedas “QM-9”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In an eleventh very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is —OCH₂CH₂CH₂CH₂CH₃, the compound ofstructure (I) then having structure (XV) (hereinafter also abbreviatedas “QM-10”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a twelfth very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is —OCH₂CH₂CH₂CH₂CH₂CH₃, the compound ofstructure (I) then having structure (XVI) (hereinafter abbreviated as“QM-11”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a thirteenth very particularly preferred embodiment of thecomposition (AB) according to the invention, the compound of structurehas (I) has R¹ and R² being tert-butyl and R³ is —C≡C-phenyl, thecompound of structure (I) then having structure (XVII) (hereinafterabbreviated as “QM-12”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fourteenth very particularly preferred embodiment of thecomposition (AB) according to the invention, the compound of structure(I) has R¹ and R² being tert-butyl and R³ is —COOH, the compound ofstructure (I) then having structure (XVIII) (hereinafter abbreviated as“QM-13”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fifteenth very particularly preferred embodiment of the composition(AB) according to the invention, the compound of structure (I) has R¹and R² being tert-butyl and R³ is —SCH₂CH₂CH₂CH₃, the compound ofstructure (I) then having structure (XIX) (hereinafter abbreviated as“QM-14”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In composition (AB) according to the invention, the total concentrationof all compounds of structure (I) in said composition (AB) maypreferably be between 10 ppb (m/m) and 100,000 ppm (m/m), morepreferably between 1 ppm (m/m) and 50,000 ppm (m/m), even morepreferably between 10 ppm and 10,000 ppm (m/m), most preferably between100 ppm and 5000 ppm (m/m), each based on the total weight of allcyclopentadiene compounds in said composition (AB).

Composition (AB) according to the invention in a further preferredembodiment may additionally also comprise (C), where “(C)” has themeaning “at least one solvent”.

Any material may be useful as a solvent in the composition (AB)according to the invention provided (A) is soluble therein in thedesired concentration range and it is both compatible with (A) and doesnot have a disruptive effect on the process according to the invention,and is in particular an apolar solvent, preferably an apolar aromatic oraliphatic solvent. It is more preferable for the solvent in thecomposition (AB) according to the invention to be selected from thegroup consisting of benzene, mono- or polyalkylated aromatics, alkaneshaving a carbon number of 6 to 15, cycloalkanes having a carbon numberof 6 to 15, high-boiling hydrocarbon cuts, ethers having a carbon numberof 6 to 15 and esters having a carbon number of 6 to 15. It is stillmore preferable for the solvent in the composition (AB) according to theinvention to be selected from the group consisting of benzene, toluene,ethylbenzene, xylene, styrene and high-boiling aromatic hydrocarboncuts. It is particularly preferable for the solvent in the composition(AB) according to the invention to be selected from the group consistingof toluene, ethylbenzene, xylene and styrene. Alternatively, thecyclopentadiene compound itself may also serve as solvent in thecomposition (AB) according to the invention.

When composition (AB) according to the invention also comprises (C), the(m/m) ratio of the total weight of all compounds of structure (I) whichare comprised by composition (AB) to the total weight of all solventscomprised by composition (AB) in composition (AB) may preferably be inthe range from 1:1000 to 100:1, more preferably in the range from 1:100to 10:1 and still more preferably in the range from 1:10 to 3:1.

Use According to the Invention

The expression “use according to the invention” is synonymous with “useof (A) for inhibiting the polymerization of (B)”.

The invention also provides for the use of (A) for inhibiting thepolymerization of (B), wherein

(A) is at least one compound of structure (1)

wherein

-   -   R¹ and R² are each independently hydrogen, alkyl of 1 to 18        carbon atoms, cycloalkyl of 3 to 15 carbon atoms, aryl of 6 to        15 carbon atoms or phenylalkyl of 7 to 15 carbon atoms;    -   R³ is —CN, —COOH, —COOR⁴, —COR⁵, —OCOR⁶, —CONR⁷R6, —PO(OR⁹)₂,        —O—R¹⁰, —S—R¹¹, —R¹², —C≡C—R¹³ or halogen;    -   wherein        -   R⁴, R⁵, R⁶ are alkyl of 1 to 18 carbon atoms, cycloalkyl of            3 to 12 carbon atoms, aryl of 6 to 10 carbon atoms;        -   R⁷ and R⁸ are each independently hydrogen; alkyl of 1 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            alkylamino having 1 to 4 carbon atoms, dialkylamino having 2            to 8 carbon atoms and hydroxyl; phenylalkyl of 7 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            hydroxyl, alkyl having 1 to 4 carbon atoms, alkylamino            having 1 to 4 carbon atoms and dialkylamino having 2 to 8            carbon atoms; aryl of 6 to 10 carbon atoms which is            unsubstituted or substituted with at least one substituent            selected from the group consisting of alkyl having 1 to 4            carbon atoms, alkylamino having 1 to 4 carbon atoms,            dialkylamino having 2 to 8 carbon atoms and hydroxyl;            -   or NR⁷R⁸ is morpholino, piperidino or pyrrolidino;        -   R⁹, R¹⁰, R¹¹ are hydrogen; alkyl of 1 to 15 carbon atoms            which is unsubstituted or substituted with at least one            substituent selected from the group consisting of hydroxyl,            dialkylamino, —OR⁴, —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3,            4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and where R¹⁴            is alkyl of 1 to 6 carbon atoms; cycloalkyl of 3 to 15            carbon atoms which is unsubstituted or substituted with at            least one substituent selected from the group consisting of            hydroxyl, dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x            is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and            where R¹⁴ is alkyl of 1 to 6 carbon atoms; phenylalkyl of 7            to 15 carbon atoms which is unsubstituted or substituted            with at least one substituent selected from the group            consisting of hydroxyl, dialkylamino, —OR¹⁴,            —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3, 4, 5, 6, 7, 8, 9 or            10 and y is 1, 2, 3 or 4 and where R¹⁴ is alkyl of 1 to 6            carbon atoms; or aryl of 6 to 15 carbon atoms which is            unsubstituted or substituted with at least one substituent            selected from the group consisting of hydroxyl,            dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3,            4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 and where R¹⁴            is alkyl of 1 to 6 carbon atoms;

R¹² is 2-pyridyl, 3-pyridyl, 4-pyridyl, 2-thienyl, 3-thienyl, 2-pyrryl,3-pyrryl, 2-furyl, 3-furyl or aryl of 6 to 15 carbon atoms; wherein theradicals 2-pyridyl, 3-pyridyl, 4-pyridyl, 2-thienyl, 3-thienyl,2-pyrryl, 3-pyrryl, 2-furyl, 3-furyl or aryl of 6 to 15 carbon atoms areunsubstituted or substituted with at least one substituent selected fromthe group consisting of hydroxyl, nitro, amino, cyano, carboxyl,aminocarbonyl, halogen, alkyl of 1 to 8 carbon atoms, alkoxy of 1 to 8carbon atoms, alkylthio of 1 to 8 carbon atoms, alkylamino of 1 to 8carbon atoms, dialkylamino of 2 to 8 carbon atoms and a carboxylic estergroup of 2 to 8 carbon atoms;

R¹³ is hydrogen, alkyl of 1 to 12 carbon atoms, aryl of 6 to 10 carbonatoms, wherein the aryl of 6 to 10 carbon atoms is unsubstituted orsubstituted with at least one substituent selected from the groupconsisting of hydroxyl, nitro, amino, cyano, carboxyl, aminocarbonyl,halogen, alkyl of 1 to 8 carbon atoms, alkoxy of 1 to 8 carbon atoms,alkylthio of 1 to 8 carbon atoms, alkylamino of 1 to 8 carbon atoms,dialkylamino of 2 to 8 carbon atoms and a carboxylic ester group of 2 to8 carbon atoms;

wherein the substituents R¹, R² and R³ are the same or different, and(B) is at least one cyclopentadiene compound.

In one preferred embodiment of the use according to the invention, R¹and R² in the compound of structure (I) are each independently selectedfrom the group consisting of methyl, ethyl, n-propyl, iso-propyl,n-butyl, iso-butyl and tert-butyl; and

R³ is —CN, —COOH, —COOR⁴, —COR⁵, —OCOR⁶, —CONR⁷R8, —PO(OR⁹)₂, —O—R¹⁰,—S—R¹¹, —R¹² —C≡C—R¹³ or halogen;

-   -   wherein    -   R⁴, R⁵, R⁶ are alkyl of 1 to 8 carbon atoms or phenyl;    -   R⁷R⁸ are each independently hydrogen or alkyl of 1 to 4 carbon        atoms, or NR⁷R⁸ is morpholino or piperidino;    -   R⁹, R¹⁰, R¹¹ are alkyl of 1 to 8 carbon atoms or phenyl;    -   R¹² is 2-furyl, 3-furyl or aryl of 6 to 15 carbon atoms, wherein        the radicals 2-furyl, 3-furyl or aryl of 6 to 15 carbon atoms        are unsubstituted or substituted with at least one substituent        selected from the group consisting of hydroxyl and alkyl of 1 to        8 carbon atoms; and    -   R¹³ is hydrogen, alkyl of 1 to 12 carbon atoms, aryl of 6 to 10        carbon atoms.

In one more preferred embodiment of the use according to the invention,R¹ and R² in the compound of structure (I) are each independentlyselected from the group consisting of methyl and tert-butyl; and

R³ is —CN, —COOH, —COOR⁴, —O—R¹⁰, —S—R¹¹, —R¹², —C≡C—R¹³ or halogen;

-   -   wherein R⁴ is alkyl of 1 to 4 carbon atoms;    -   R¹⁰, R^(1′) are alkyl of 1 to 6 carbon atoms;    -   R¹² is 2-furyl, 3-furyl or aryl of 6 to 12 carbon atoms, wherein        the radicals 2-furyl, 3-furyl or aryl of 6 to 12 carbon atoms        are unsubstituted or substituted with at least one substituent        selected from the group consisting of hydroxyl and alkyl of 1 to        8 carbon atoms; and    -   R¹³ is aryl of 6 to 10 carbon atoms.

In one still more preferred embodiment of the use according to theinvention, R¹ and R² are each tert-butyl in the compound of structure(I); and R³ is —CN, —COOH, —O—R¹⁰, —S—R¹¹, —R¹² or —C≡C—R¹³;

-   -   wherein    -   R¹⁰ is methyl, ethyl, iso-propyl, n-propyl, n-butyl, iso-butyl,        sec-butyl, tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,        3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,        2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl,        2-methylpentyl, 3-methylpentyl, 4-methylpentyl,        1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl,        2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl,        1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl,        1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl or        1-ethyl-2-methylpropyl, preferably methyl, ethyl, iso-propyl,        n-propyl, sec-butyl, n-butyl, n-pentyl, or n-hexyl;    -   R¹¹ is alkyl of 1 to 6 carbon atoms;    -   R¹² is 2-furyl, 3-furyl or phenyl, wherein phenyl is        unsubstituted or substituted with at least one substituent        selected from the group consisting of hydroxyl and alkyl of 1 to        8 carbon atoms; and    -   R¹³ is phenyl.

In a first very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is CN, the compound of structure (I) then havingstructure (V) (hereinafter also abbreviated as “QM-1”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a second very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is phenyl, the compound of structure (I) then havingstructure (VI) (hereinafter also abbreviated as “QM-2”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a third very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is 3,5-di-tert-butyl-4-hydroxyphenyl, the compound ofstructure (I) then having structure (VII) (hereinafter also abbreviatedas “QM-3”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fourth very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is 2-furyl, the compound of structure (I) then havingstructure (VIII) (hereinafter also abbreviated as “QM-4”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fifth very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is —OR¹⁰, where R¹⁰ is methyl, ethyl, iso-propyl,n-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl,1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1,1-dimethylpropyl,1,2-dimethylpropyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl,1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl,1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl,2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl,2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl,1-ethyl-1-methylpropyl or 1-ethyl-2-methylpropyl, preferably methyl,ethyl, iso-propyl, n-propyl, n-butyl, sec-butyl, tert-butyl, n-pentyl orn-hexyl, more preferably methyl, ethyl, n-propyl, iso-propyl, n-butyl,n-pentyl or n-hexyl;

the compound of structure (I) then having structure (IX)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a sixth very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³═—OCH₃, the compound of structure (I) then havingstructure (X) (hereinafter also abbreviated as “QM-5”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a seventh very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is —OCH₂CH₃, the compound of structure (I) then havingstructure (XI) (hereinafter also abbreviated as “QM-6”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In an eighth very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is —OCH₂CH₂CH₃, the compound of structure (I) thenhaving structure (XII) (hereinafter also abbreviated as “QM-7”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a ninth very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is —OCH(CH₃)₂, the compound of structure (I) thenhaving structure (XIII) (hereinafter also abbreviated as “QM-8”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a tenth very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is —OCH₂CH₂CH₂CH₃, the compound of structure (I) thenhaving structure (XIV) (hereinafter also abbreviated as “QM-9”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In an eleventh very particularly preferred embodiment of the useaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —OCH₂CH₂CH₂CH₂CH₃, the compound of structure(I) then having structure (XV) (hereinafter also abbreviated as “QM-10”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a twelfth very particularly preferred embodiment of the use accordingto the invention, the compound of structure (I) has R¹ and R² beingtert-butyl and R³ is —OCH₂CH₂CH₂CH₂CH₂CH₃, the compound of structure (I)then having structure (XVI) (hereinafter abbreviated as “QM-11”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a thirteenth very particularly preferred embodiment of the useaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —C≡C-phenyl, the compound of structure (I)then having structure (XVII) (hereinafter abbreviated as “QM-12”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fourteenth very particularly preferred embodiment of the useaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —COOH, the compound of structure (I) thenhaving structure (XVIII) (hereinafter abbreviated as “QM-13”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

In a fifteenth very particularly preferred embodiment of the useaccording to the invention, the compound of structure (I) has R¹ and R²being tert-butyl and R³ is —SCH₂CH₂CH₂CH₃, the compound of structure (I)then having structure (XIX) (hereinafter abbreviated as “QM-14”)

and the cyclopentadiene compound is selected from the group consistingof cyclopentadiene, dicyclopentadiene, alkylated cyclopentadiene andalkylated dicyclopentadiene, more preferably selected from the groupconsisting of cyclopentadiene and dicyclopentadiene.

(A) may be used in the use according to the invention in gaseous form,as a solid material (as a powder, for example) or as a liquid, inparticular as a solid material (as a powder, for example) or as aliquid, preferably as a liquid. (A) used as a liquid in the useaccording to the invention is more particularly used as a melt or in theform of a solution in (C), where “(C)” has the meaning “at least onesolvent”.

Any material may be useful as a solvent in the use according to theinvention provided (A) is soluble therein in the desired concentrationrange and it is both compatible with (A) and does not have a disruptiveeffect on the use according to the invention, and is in particular anapolar solvent, preferably an apolar aromatic or aliphatic solvent. Itis more preferable for the solvent in the use according to the inventionto be selected from the group consisting of benzene, mono- orpolyalkylated aromatics, alkanes having a carbon number of 6 to 15,cycloalkanes having a carbon number of 6 to 15, ethers having a carbonnumber of 6 to 15, high-boiling hydrocarbon cuts and esters having acarbon number of 6 to 15. It is still more preferable for the solvent inthe use according to the invention to be selected from the groupconsisting of benzene, toluene, ethylbenzene, xylene, styrene andhigh-boiling aromatic hydrocarbon cuts. It is particularly preferablefor the solvent in the use according to the invention to be selectedfrom the group consisting of toluene, ethylbenzene, xylene and styrene.Alternatively, the cyclopentadiene compound itself can also serve assolvent in the use according to the invention.

When (A) is used in the use according to the invention in the form of asolution in (C), the total weight of all compounds of structure (I) insolution (AC) preferably has an (m/m) ratio to the total weight of allsolvents in solution (AC) in the range from 1:1000 to 100:1, morepreferably in the range from 1:100 to 10:1 and still more preferably inthe range from 1:10 to 3:1.

(B) may be present in the use according to the invention in gaseousform, as a liquid or as a solid material, in particular in gaseous formor as a liquid, preferably as a liquid. (B) as a liquid is still morepreferably in the form of a melt or solution. It is particularlypreferable for (B) to be in the form of a solution. Such a solution in afirst very particularly preferred embodiment of the use according to theinvention is a process stream as obtained in cracking processes. (B) istypically present in such a process stream at from 0.0001 wt % to 15 wt%.

In an alternative second very particularly preferred embodiment of theuse according to the invention, the solution may be a process stream asgenerated in the production of DCPD and/or CPD itself. (B) is typicallypresent in such a process stream at between 15 and 100 wt %, preferablybetween 70 and 100 wt % and still more preferably between 70 and 99.99wt %.

The use according to the invention typically comprises bringing (A) and(B) into contact, which for the purposes of the invention is to beunderstood as meaning in particular that (A) is admixed to (B) or (B) isadmixed to (A).

Admixing (A) to (B) or (B) to (A) can be conducted according toconventionally known methods.

(A) can be admixed with advantage in the use according to the inventioninto any feedstream or outflow line of a distillation column, into thein- and outflow line of a heat exchanger or into the in- and outflowline of a vaporizer (“reboiler”) or into the in- and outflow line of acondenser or into the in- and outflow line of a reactor. (A) can also beadded in the use according to the invention to storage tanks containinga process stream comprising (B). (A) can be admixed to (B) not onlybefore but also during a process, for example a production orpurification process.

An effective amount of (A) is admixed in the use according to theinvention. The term “effective amount of (A)” in the context of thisinvention is to be understood as meaning the amount of (A) needed todelay/prevent the undesired polymerization of (B). This effective amountdepends on the conditions under which the cyclopentadiene compound, ormixture of two or more cyclopentadiene compounds, is stored or handledand may readily be determined from case to case by a person skilled inthe art. For example, the cracking of dicyclopentadiene requires byreason of the higher temperatures a higher amount of (A) than thestoring of (B) at for instance room temperature.

(A) is preferably used in the use according to the invention in such anamount that the total concentration of all compounds of structure (I) isbetween 10 ppb (m/m) and 100,000 ppm (m/m), more preferably between 1ppm (m/m) and 50,000 ppm (m/m), even more preferably between 10 ppm and10 000 ppm (m/m) most preferably between 100 ppm and 5000 ppm (m/m),each based on the total weight of all cyclopentadiene compounds.

The temperature at which the use according to the invention can becarried out is not subject to any in-principle limitation; on thecontrary, it is a feature of the present invention that the useaccording to the invention can be carried out not only at low but alsoat high temperatures, in particular in the range from 0° C. to 250° C.,preferably 0° C. to 200° C.

The use according to the invention may utilize a polymerizationinhibitor (D) as well as (A). Polymerization inhibitors of this type areconventionally known, examples being nitroxides such as, for instance,oxo-TEMPO or 4-hydroxy-TEMPO, phenylenediamines, hydroxylamines such asdiethylhydroxylamine (DEHA), nitro- or nitrosoaromatics such as DNBP,(di)phenols such as hydroquinone, TBC or 2,6-di-tert-butylphenol,benzoquinones, phenothiazines such as PTZ.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples, which areprovided herein for purposes of illustration only, and are not intendedto be limiting unless otherwise specified.

The examples which follow shall further elucidate the invention withoutthe invention being limited to these embodiments.

EXAMPLES General Description Screening Test Examples 1-11

The following apparatus was set up: A 250 mL multi-neck flask was fittedwith a reflux condenser, a nitrogen supply and a sampler.

A 100 g quantity of dicyclopentadiene (purity: 98%) was melted andweighed into the 250 mL flask.

Nitrogen was passed over the dicyclopentadiene, and 50 mg (500 ppm) ofthe in-test inhibitor shown in Table 1 are added.

While the flow of nitrogen over the dicyclopentadiene was continued, theflask was immersed in a preheated oil bath at 170° C. As the flask wasimmersed, the reaction started.

Beginning with the immersion of the flask, 0.5-1 mL samples were takenevery 30 minutes with a glass syringe. The samples were diluted in a 1:9weight ratio in ethylbenzene and measured using an evaporative lightscattering (ELS) detector.

The ELS detector (Polymerlabs; model: PL-ELS 1000) was connected to anHPLC system which was operated without separation column. Ethylbenzenewas used as the mobile phase at a flow rate of 1 mL/min. The injectionvolume of the diluted sample is 20 μL.

The ELS detector was set to the following parameters:

-   -   nitrogen stream: 1.2 l/h    -   nebulizer: 100° C.    -   evaporator: 130° C.

The peak area detected was a measure of the oligomer/polymer content ofthe sample. The oligomer/polymer contents determined are not absolute.The peak area was proportional to the oligomer/polymer content in themeasured region, so the results of the various inhibitors arecomparable.

The results after 120 min and 240 min are summarized below in Table1—and all measured values are depicted in graphical form in FIG. 1.

Example 1 is the blank (without admixture of an inhibitor). Examples 2to 6 are comparative examples, not in accordance with the invention,which were carried out with conventionally known cyclopentadienepolymerization inhibitors 4-hydroxy-TEMPO (4-HT; Example 2),4-butoxy-TEMPO (4-BT; Example 3), tert-butylcatechol (TBC; Example 4)and dinitro-sec-butylphenol (DNBP; Example 5), hydroquinone monomethylether (MeHQ; Example 6). Examples 7 to 11 are examples in accordancewith the invention which were carried out with the compounds QM-1(Example 7), QM-2 (Example 8), QM-5 (Example 9), QM-7 (Example 10) andQM-11 (Example 11).

TABLE 1 Peak area Ex- after after am- 120 240 ple Name and structure ofinhibitor min min 1 no inhibitor (blank) 1100 2300 2

 810  1880 3

 750 1830 4

 340  930 5

1140 2450 6

1110 2350 7

 90  320 8

 160  290 9

 160  360 10

 200  450 11

 330  660Results regarding Examples 1-11:

Example 1 (Comparative Example, not in Accordance with the Invention):Blank Value (without Inhibitor Admixture)

The curve slopes up continuously over the measurement period of 4 hours,i.e. the polymer content increased continuously. Peak area was 1100 whenmeasured after two hours, slightly more than doubling to 2300 after fourhours.

Examples 2 & 3 (Comparative Examples, not in Accordance with theInvention): TEMPO derivatives (4-hydroxy-TEMPO; 4-butoxy-TEMPO)

The curve in FIG. 1 shows that almost no polymer formed in the first 30minutes, but thereafter the curves (and hence the polymer content) slopeup at the same slope as the curve of the blank test. A peak area of1400-1700 was attained in this way after four hours.

Example 4 (Comparative Example, not in Accordance with the Invention):TBC

Polymerization was slowed greatly compared with the blank, the TEMPOderivatives and/or DNBP. Peak area even after four hours is just 640.

Example 5 (Comparative Example, not in Accordance with the Invention):DNBP

DNBP is a good retarder with “normal” polymerization-pronevinyl-containing monomers, e.g. styrene (see Example 16, not inaccordance with the invention). DNBP is likewise reputed to intervene inthe Diels-Alder mechanism. DNBP should therefore have been expected todo well in this test. Yet, when used in the test, DNBP was found to haveno effect in relation to the polymerization of cyclopentadiene ordicyclopentadiene. Polymerization proceeded in the presence of DNBP inexactly the same way as without any admixture. Detected peak area aftertwo and/or four hours corresponded to that of the blank value (Example1).

Example 6 (Comparative Example, not in Accordance with the Invention):MeHQ

MeHQ was used as stabilizer in some of the processes described in theliterature. Yet tested with (di-)cyclopentadiene it showed no effect.

Examples 7-11 (Examples in Accordance with the Invention): QuinoneMethides [QM-1, QM-2, QM-5, QM-7, QM-11]

The quinone methides tested are good retarders—as good as DNBP—with“normal” polymerization-prone vinyl-containing monomers, e.g. styrene.Based on conventional wisdom as reported in the literature DNBP v.quinone methides in styrene—quinone methides would therefore not beexpected to show any activity.

It was all the more astonishing that the quinone methides used have avery substantial slowing effect on the polymerization ofcyclopentadiene/dicyclopentadiene. The effect was greater than that ofany other of the inhibitors tested.

Comparative Test with Styrene Monomer Examples 12-21

Commercially available stabilized styrene was freed of the stabilizertert-butyl-1,2-hydroxybenzene (TBC) in an inert nitrogen atmosphere at areduced pressure of 95 mbar and a pot temperature of 75° C. Theexperimental apparatus, which consisted of a multi-neck flask equippedwith a thermometer, a reflux condenser, a septum and a KPG stirrer, wasthoroughly purged with nitrogen to obtain an oxygen-free atmosphere. 300g of the unstabilized styrene were introduced into the multi-neck flaskand admixed with 100 ppm of an inhibitor as per Table 2. A constantsupply of nitrogen into the styrene solution through a glass fritensured an inert nitrogen atmosphere throughout the entire duration ofthe experiment. The styrene solution was vigorously stirred.

At the start of the experiment, the flask was immersed in a preheatedoil bath at 110° C. to such an extent that the stabilized styrenesolution is completely immersed. After the three-neck flask had beenimmersed in the heated oil bath, about 3 g of the styrene solution wereremoved via the septum at regular intervals, accurately weighed andintroduced into 50 ml of methanol. The methanol mixture was stirred atroom temperature for half an hour. The methanol worked to precipitatethe polystyrene formed during the experiment. This polystyrene wasseparated off by filtration through a glass filter crucible. The filterresidue was washed with 20 ml of methanol and then dried at 110° C. fornot less than 5 hours. The polystyrene remaining behind in the glassfilter crucible was then weighed. The value found and the initial weightwere used to determine the percentage fraction of polymer. This polymercontent was plotted against the reaction time (cf. also further valuesdepicted in FIG. 2).

TABLE 2 Name and Polymer content in % Example structure of inhibitorafter 120 min after 210 min 12 no inhibitor (blank) 9.3 16.4 134-hydroxy-TEMPO (4-HT) 3.0 8.2 14 4-butoxy-TEMPO 2.4 8.1 15 TBC 6.0 14.216 DNBP 1.2 2.8 17 QM-1 0.9 10.3 18 QM-2 1.0 2.9 19 QM-5 1.1 2.8 20 QM-72.0 3.4 21 QM-8 2.7 5.0

Evaluation of Examples 12-21

It is apparent from the table that the TEMPO derivatives (Examples 13and 14) were effective inhibitors of the polymerization of styrene—for ashort time. Thereafter, they were spent and virtually devoid of anyfurther activity. Corresponding results were found in(di)cyclopentadiene (Examples 2 and 3).

TBC (Example 15), however, had virtually no effect in the styrene test,but was found to have fairly good activity in (di)cyclopentadiene(Example 4).

With DNBP (Example 16), the effect was exactly the other way round.While its performance in styrene was virtually equivalent to or evenbetter than that of the quinone methides QM-2, QM-5, QM-7, QM-8(Examples 18-21), DNBP had no effect in (di)cyclopentadiene (Example 5).Yet all the quinone methides were very active in (di)cyclopentadiene(Examples 7-11). In contrast to the other quinone methides, QM-1 provedto be a potent inhibitor in styrene, but was quick to lose its activity(Example 17). In (di)cyclopentadiene, by contrast, it surprisingly showsvery good, sustained activity (Example 7).

Comparing the results of the tests with (di)cyclopentadiene and styrenesuggests that different mechanisms are involved in the polymerization ofthe two unsaturated monomers. The effectiveness of inhibitors cannot bepredicted as to between monomers.

The values obtained with QM-1 and QM-5 are shown for comparison in FIG.2.

General Description—Use in Cyclopentadiene Production Examples 22 and 23

A 500 ml multi-neck flask was volumetrically calibrated and marked at400 mL. Continuous metered addition of fresh dicyclopentadiene wasprovided. The flask was fitted with a heatable column packed with glassRaschig rings. The oil bath was temperature regulated to 180° C.

Technical-grade DCPD (93%) was used in the continuous runs.

The entire dicyclopentadiene to be used was admixed with 5000 ppm of thein-test inhibitor. A slow stream of nitrogen was passed continuouslyover 100 g of dicyclopentadiene (with inhibitor).

The multi-neck flask was then immersed in the preheated oil bath. Once apot temperature of 160° C. was reached, 30 ml per hour ofdicyclopentadiene (with inhibitor) were metered continuously into thepot. The pot begins to boil at a temperature of about 164° C. and thedicyclopentadiene was cleaved into cyclopentadiene, which distilled overthrough the column. The cyclopentadiene produced was collected in areceiver cooled to −3° C.

Once insufficient cyclopentadiene was formed under the giventemperatures, the pot level rises. When a pot level of 400 mL wasreached, the metered addition was terminated and remainingdicyclopentadiene and cyclopentadiene formed was distilled out of thepot. The results are shown in Table 3.

TABLE 3 Run time Total amount Isolated Yield Example Inhibitor in h ofDCPD in g CPD in g in % 22 no addition 38.00 1197.2 854.8 76.8 23 QM-543.50 1324 976.8 79.3

The table reveals that the addition of QM-5 had a distinct prolongatingeffect on the run time in the production of dicyclopentadiene andcyclopentadiene. Instead of for 38 h, the apparatus could be operatedfor 43.5 h without pot exchange. In addition, the cyclopentadiene yield,based on the entire feed of dicyclopentadiene, went up from 76.8% to79.3%.

1. A process for inhibiting the polymerization of a cyclopentadienecompound (B), comprising: contacting the cyclopentadiene compound (B)with at least one compound (A); wherein (A) is a compound of structure(I)

wherein R¹ and R² are each independently hydrogen, alkyl of 1 to 18carbon atoms, cycloalkyl of 3 to 15 carbon atoms, aryl of 6 to 15 carbonatoms or phenylalkyl of 7 to 15 carbon atoms; R³ is —CN, —COOH, —COOR⁴,—COR⁵, —OCOR⁶, —CONR⁷R⁸, —PO(OR⁹)₂, —O—R¹⁰, —S—R¹¹, —R¹², —C≡C—R¹³ orhalogen; wherein R⁴, R⁵, R⁶ are alkyl of 1 to 18 carbon atoms,cycloalkyl of 3 to 12 carbon atoms, aryl of 6 to 10 carbon atoms; R⁷ andR⁸ are each independently hydrogen; alkyl of 1 to 15 carbon atoms whichis optionally substituted with at least one substituent selected fromthe group consisting of alkylamino having 1 to 4 carbon atoms,dialkylamino having 2 to 8 carbon atoms and hydroxyl; phenylalkyl of 7to 15 carbon atoms which is optionally substituted with at least onesubstituent selected from the group consisting of hydroxyl, alkyl having1 to 4 carbon atoms, alkylamino having 1 to 4 carbon atoms anddialkylamino having 2 to 8 carbon atoms; aryl of 6 to 10 carbon atomswhich is optionally substituted with at least one substituent selectedfrom the group consisting of alkyl having 1 to 4 carbon atoms,alkylamino having 1 to 4 carbon atoms, dialkylamino having 2 to 8 carbonatoms and hydroxyl; or NR⁷R⁸ is morpholino, piperidino or pyrrolidino;R⁹, R¹⁰, R¹¹ are hydrogen; alkyl of 1 to 15 carbon atoms which isoptionally substituted with at least one substituent selected from thegroup consisting of hydroxyl, dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H,where x is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 and y is 1, 2, 3 or 4 andwhere R¹⁴ is alkyl of 1 to 6 carbon atoms; cycloalkyl of 3 to 15 carbonatoms which is optionally substituted with at least one substituentselected from the group consisting of hydroxyl, dialkylamino, —OR¹⁴,—[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 and y is1, 2, 3 or 4 and where R¹⁴ is alkyl of 1 to 6 carbon atoms; phenylalkylof 7 to 15 carbon atoms which is optionally substituted with at leastone substituent selected from the group consisting of hydroxyl,dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x is 1, 2, 3, 4, 5, 6, 7,8, 9 or 10 and y is 1, 2, 3 or 4 and where R¹⁴ is alkyl of 1 to 6 carbonatoms; or aryl of 6 to 15 carbon atoms which is optionally substitutedwith at least one substituent selected from the group consisting ofhydroxyl, dialkylamino, —OR¹⁴, —[O(CH₂)_(y)]_(x)H, where x=1, 2, 3, 4,5, 6, 7, 8, 9 or 10 and y=1, 2, 3 or 4 and where R¹⁴=alkyl of 1 to 6carbon atoms; R¹² is 2-pyridyl, 3-pyridyl, 4-pyridyl, 2-thienyl,3-thienyl, 2-pyrryl, 3-pyrryl, 2-furyl, 3-furyl or aryl of 6 to 15carbon atoms; wherein the radicals 2-pyridyl, 3-pyridyl, 4-pyridyl,2-thienyl, 3-thienyl, 2-pyrryl, 3-pyrryl, 2-furyl, 3-furyl or aryl of 6to 15 carbon atoms are optionally substituted with at least onesubstituent selected from the group consisting of hydroxyl, nitro,amino, cyano, carboxyl, aminocarbonyl, halogen, alkyl of 1 to 8 carbonatoms, alkoxy of 1 to 8 carbon atoms, alkylthio of 1 to 8 carbon atoms,alkylamino of 1 to 8 carbon atoms, dialkylamino of 2 to 8 carbon atomsand a carboxylic ester group of 2 to 8 carbon atoms; R¹³ is hydrogen,alkyl of 1 to 12 carbon atoms, aryl of 6 to 10 carbon atoms, wherein thearyl of 6 to 10 carbon atoms is optionally substituted with at least onesubstituent selected from the group consisting of hydroxyl, nitro,amino, cyano, carboxyl, aminocarbonyl, halogen, alkyl of 1 to 8 carbonatoms, alkoxy of 1 to 8 carbon atoms, alkylthio of 1 to 8 carbon atoms,alkylamino of 1 to 8 carbon atoms, dialkylamino of 2 to 8 carbon atomsand a carboxylic ester group of 2 to 8 carbon atoms; and wherein thesubstituents R¹, R² and R³ are the same or different.
 2. The processaccording to claim 1, wherein R¹ and R² are each tert-butyl in thecompound of structure (I), and R³ is —CN, —COOH, —O—R¹⁰, —S—R¹¹, —R¹² or—C≡C—R¹³; wherein R¹⁰ is methyl, ethyl, iso-propyl, n-propyl, n-butyl,iso-butyl, sec-butyl, tert-butyl, n-pentyl, 1-methylbutyl,2-methylbutyl, 3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl,2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl,1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl,2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl,1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl or1-ethyl-2-methylpropyl; R¹¹ is alkyl of 1 to 6 carbon atoms; R¹² is2-furyl, 3-furyl or phenyl, wherein phenyl is optionally substitutedwith at least one substituent selected from the group consisting ofhydroxyl and alkyl of 1 to 8 carbon atoms; and R¹³ is phenyl.
 3. Theprocess according to claim 1, wherein R¹ and R² are each tert-butyl inthe compound of structure (I) and R³ is —O—R¹⁰ where R¹⁰=methyl, ethyl,n-propyl, iso-propyl, n-butyl, n-pentyl or n-hexyl.
 4. The processaccording to claim 1, wherein R¹ and R² are each tert-butyl in thecompound of structure (I) and R³ is —R¹² where R¹² is 2-furyl, 3-furylor phenyl.
 5. The process according to claim 1, wherein thecyclopentadiene compound is at least one selected from the groupconsisting of cyclopentadiene, dicyclopentadiene, alkylatedcyclopentadiene and alkylated dicyclopentadiene.
 6. The processaccording to claim 1, wherein the cyclopentadiene compound is at leastone of cyclopentadiene and dicyclopentadiene.
 7. The process accordingto claim 1, wherein (A) is in the form of a solution (AC) in (C),wherein (C) is at least one solvent selected from the group consistingof benzene, a mono-alkylated aromatic compound, a polyalkylated aromaticcompound, an alkane having a carbon number of 6 to 15, a cycloalkanehaving a carbon number of 6 to 15, a high-boiling hydrocarbon cut, anether having a carbon number of 6 to 15 and an ester having a carbonnumber of 6 to
 15. 8. The process according to claim 7, wherein a ratio(m/m) of a total weight of all compounds of structure (I) in solution(AC) to a total weight of all solvents in solution (AC) is from 1:1000to 100:1.
 9. The process according to claim 1, wherein a totalconcentration of all compounds of structure (I) is between 10 ppb (m/m)and 100,000 ppm (m/m), based on the total weight of all cyclopentadienecompounds.
 10. The process according to claim 1, wherein (B) is acomponent of a process stream and a concentration of (B) is 0.0001 to 15wt %. 11-17. (canceled)